A conventionally known pump structure used in a liquid chromatograph includes two plunger pumps which are connected in series or in parallel in such a manner that a first plunger pump and a second plunger pump perform coordinated operations to reduce pulsations (see Patent Document 1, for example). The configuration disclosed in Patent Document 1, for instance, is the configuration in which the first plunger pump and the second plunger pump are connected in series and reciprocating motions of the plungers are driven by use of cams. In phase 1 of a pumping operation, a downstream pump 1a performs an intake action (at an ejection flow rate of 0) while an upstream pump 1b performs ejection at a flow rate of Q, and a total ejection amount is therefore equal to Q. In phase 2, both of the pumps perform ejection at a flow rate of Q/2, and the total ejection amount is therefore equal to Q. In phase 3, the upstream pump 1b performs an intake action at a flow rate of 3Q/2 while the downstream pump 1a performs an ejection action at a flow rate of 3Q/2+Q, and the total ejection amount is therefore equal to Q. In this way, the pulsations are reduced by stably feeding a liquid while maintaining the total flow rate of Q at any time regardless of the intake actions of the pumps.
Meanwhile, there is also proposed a technique for suppressing occurrence of pulsations by controlling the number of revolutions of a motor for driving a plunger by use of values detected by a plunger-position detection sensor and a flow rate sensor (see Patent Document 2, for example).
However, the inventions described in Patent Document 1 and Patent Document 2 do not take into consideration the processing accuracy, assembly accuracy, and control accuracy of the pumps. For example, a cyclic pulsation occasionally occurs due to a backlash or a dimensional looseness in a cam mechanism for driving a plunger pump, manufacturing accuracy, vibration at the time of a change in speed, or other factors. A pulsation thus caused leads to a fluctuation in a flow rate of a solvent that passes through a separation column in a liquid chromatograph or a mixing ratio of a solvent, and to deterioration in accuracy of a detection result shown on a chromatogram. In this regard, there has been a demand for a technique for suppressing a pressure pulsation of a pump for liquid chromatograph while taking into consideration an actual device configuration.
Another technique is known in which a mixer or a pulsation absorbing damper is provided to piping, so that a pulsation caused by a pump in a liquid chromatograph is not transmitted to a separation column (see Patent Document 3, for example). Although such devices are effective in attenuating a pulsation at a specific cycle, the devices can hardly reduce pulsations of all cycles because a pump for liquid chromatograph, which is a subject of the present invention, is configured to control an ejection flow rate by changing a drive cycle.